In some conventional exhaust gas recirculation systems for an internal combustion engine, the rate of exhaust gas to be recirculated is controlled by an electromagnetic valve responsive to a driving pulse signal, where the duty cycle of the driving pulse signal is controlled in accordance with various engine parameters such as the flow rate of the intake air of the engine, and the rotational speed of the crankshaft of the engine.
As the power source of the driving pulse signal, a battery is usually used. If the voltage of the battery is constant, the flow rate of the gas is proportionally controlled via the electromagnetic valve in accordance with the calculated duty cycle of the driving pulse signal. However, the voltage of the battery is apt to vary in a considerably wide range. When the voltage of the driving pulse signal of the electromagnetic valve varies, the response of the electromagnetic valve varies accordingly. In other words, the actual duty cycle of the operation (the period of time for which the valve opens with respect to the duration of one cycle of open and close states) of the electromagnetic valve does not correspond with the duty cycle of the driving pulse signal. This means that, when the voltage of the power supply is different from the standard voltage thereof, the recirculation rate of the exhaust gas differs from a desired rate which is expected, although the duty cycle of the driving pulse signal is correctly controlled.